Enhancing the Efficiency of Polymer Solar Cells by Incorporation of 2,5-Difluorobenzene Units into the Polymer Backbone via Random Copolymerization

A series of conjugated polymers P0, P5, and P7 containing 0, 5, and 7 mol % 2,5-difluorobenzene units, respectively, were prepared and utilized as electron donors in polymer solar cells. Incorporation of a small amount of 2,5-difluorobenzene unit into the backbone of donor polymers can significantly increase their planarity and crystallinity as well as decrease their solubility. The improved molecular conformation can markedly affect the morphology of polymer:PC71BM blend films. After incorporation of 5 mol % 2,5-difluorobenzene unit into the backbone of donor polymers, the domain size of blend films became smaller and the hole mobility increased. Increasing the content of 2,5-difluorobenzene to 7 mol % can further decrease the solubility of resulting polymers and resulted in poor solution processability. As a result, P5-based devices achieved a power conversion efficiency (PCE) of 8.5%, whereas P0 based devices gave a PCE of 7.8%.